We propose to study the acquisition of locomotor skills in infancy and to examine the relations between locomotor and psychological development during the transition from prelocomotion to self-produced prone locomotion (e.g., creeping and crawling). Our specific aims are to: (a) develop quantitative measures of the organization of skills that underlie the acquisition of self-produced locomotion; and (b) use these measures to examine the relations between the acquisition of locomotor skill and shifts in spatial, cognitive and perceptual development. Two equally important and related sets of studies are proposed. The first set of five studies will provide basic information about the motoric processes that underlie the acquisition of self-produced locomotor skills. Study 1 is a normative longitudinal study of the transition from prelocomotion to the acquisition of prone self-produced locomotion. Study 2 extends study 1 by comparing our measures of locomotor skill with the judgments of pediatric physical therapists to determine if our quantitative measures of self-produced locomotion captures aspects of this skill which are important to practitioners. In studies 3 - 5 we will experimentally manipulate locomotor-skill performance to determine how locomotor skills are organized and reorganized when interventions are introduced that serve to impede or facilitate locomotion. The second set of three studies are designed to determine the extent to which important psychological changes follow in time, coincide with, are mediated by, or are maintained by the acquisition of locomotor skills. Study 6 will examine the relation between active and passive locomotor experience. The question is how sensorimotor-based knowledge affects spatial relations in an object search task. Study 7 will test whether self-produced locomotion is the mechanism by which self-movement is discriminated from world-movement when vestibular and visual information are placed in conflict in the context of a "moving room." Study 8 will examine how locomotor experience facilitates the baby's understanding of distance and size-constancy relations in order to coordinate perspectival transformations.